I"m knitting the Russian coat from Vogue magazine. It's a series of hexagons attached to a beginning triangle. I assume whomever will answer this question knows this technique I'm talking about and doesn't need to know much more than that. If so, I will certainly add to this post.

So, I casted on my first 131 stitches to an already existing triangle, worked the hexagon by decreasing in the pattern. I sewed the edges and created a hexagon. The first hexagon looks pretty much perfect, nice and flat when you place it on the table.

On to the second hexagon, and when I sewed it together, it puffs up in the center when it's put flat on the table. The other one didn't do that, and I'm wondering why this one is. It's pretty much a stockinette stitch, easy to count rows, and there is the correct amount of rows, correct decreasing throughout.

My gauge is good; I went down two needles sizes, but right on with number of stitches and rows.

Did I get "comfortable" with this hexagon and maybe tighten up a bit, and that's what caused it to pucker up?

Could be you got tighter, usually people relax as they get 'comfortable' with a pattern, though daily stress can cause your gauge to be tighter. You need to check it as you go along, not just rely that you're getting the same gauge as the first pieces. You said you sewed it on, so you must have bound it off - is the BO too tight?

Actually, the way this ends is not with binding off. I keep decreasing until 14 stitches left, then I cut yarn leaving about 12 inches and use a crochet hook to pull that yarn through those 14 sts and just pull tight.

I think, though, it is possible that I tightened back up, then, because I generally knit tighter rather than loose, but tried to loosen up a bit.